the.com/cold spell

means A stretch of days when the temperature drops well below normal and stays there, long enough to feel like weather turned into a sentence.

from Straightforward compound English, not sorcery: 'cold' plus 'spell,' where 'spell' means a continuous stretch of time or activity, a sense traced back to Old English 'spelian' (to take the place of, to stand in for one's turn) — the same root that gave us a 'spell of work' or someone 'spelling' you at a task. Over time 'spell' drifted from 'a turn at doing something' to just 'a period during which something holds,' so a cold spell is simply cold weather taking its turn and refusing to be relieved.